This invention relates to the synthesis of novel hydroxy polyimides and their application in making aqueous base developable positive resists for high temperature applications.
Positive resist compositions and their use are well known. Generally such resists compositions are prepared from alkali-soluble, phenol-formaldehyde novolak resins and light or radiation sensitive o-quinone diazides or naphthoquinone diazides. Examples of such positive resists are described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470.
Conventional positive novolak resists have limited temperature dimensional stability and are not well suited for modern high temperature processes and applications. Heat resistant, negative resists are known in the art and disclosed in U.S. Pat. Nos. 3,957,512; 4,045,223; 4,088,489; Re. 30,186 and DE 3411659Al. Also heat resistant, positive resists are known and disclosed in U.S. Pat. Nos. 4,093,461; 4,339,521; and 4,395,482. The advantages of positive resists over negative resists are also known in the art which include higher resolution and exposure time stability in the presence of oxygen.
In the industrial application of positive resists: the polymeric component and the radiation sensitizer are dissolved in an organic solvent or mixture of solvents and applied as a thin film or coating to a substrate suitable for the desired application.
The polymeric component of these resist formulations is desirably soluble in aqueous alkaline solutions, but the sensitizer acts as a dissolution rate inhibitor with respect to the polymer. Upon exposure of selected areas of the coated substrate to actinic radiation, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the resist coating to be dissolved when the substrate is immersed in developing solution leaving the unexposed areas substantially intact, thus producing a positive relief pattern on the substrate.
In most applications, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The resist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive resist, corresponds to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern of the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of resist on substrate produced by the method described above is useful for various applications including, for example, as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components or the manufacture of a printing plate.
The properties of a resist composition which are important in commercial practice include the solubility of the resist in the application solvent, the photospeed of the resist, development contrast, environmently acceptable developer solubility, resist resolution, resist adhesion, dimensional stability at elevated temperature and abrasion resistance.
Photospeed is important for a resist, particularly in applications where a number of exposures are needed, for example, in generating multiple patterns by a repeated process, or where light of reduced intensity is employed such as in projection exposure techniques where the light is passed through a series of lenses and mono-chromatic filters. Thus, high, controlled photospeed is particularly important for a resist composition employed in processes where a number of multiple exposures must be made to produce a mask or series of circuit patterns on a substrate. Control of the photospeed is extremely important in order to produce high resolution relief patterns in microcircuitry; e.g., a photospeed too high can result in narrowing the processing conditions.
Resist resolution refers to the capability of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces. In many industrial applications, particularly in the manufacture of miniaturized electronic components, a resist is required to provide a high degree of resolution for very small line and space widths (on the order of a micron or so).
The ability of a resist to reproduce very small dimensions, on the order of a micron or so, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Various attempts have been made in the prior art to produce high temperature positive resists possessing the above desired properties. For example, U.S. Pat. No. 4,093,461 discloses a heat resistant, positive resist composition comprising a quinone or naphthoquinone diazide and the polycondensation product of an aromatic dianhydride (pryomellitic anhydride) and an aromatic diamine (4,4'-diaminodiphenylether). The properties of the positive resist of the patent are discussed in U.S. Pat. No. 4,395,482 (column 1, lines 46-64). There it is pointed out that the positive resist composition of U.S. Pat. No. 4,093,461 has limited storage life, insufficient stability to alkaline etching solutions and relatively small differences in solubility between the exposed and unexposed portion of the resist.
To date, the use of imagable polyimide resist system has been limited by the lack of photospeed (slow photospeed), excessive volume contraction and by shelf life problems. The exceptional dielectric and high temperature resistance properties of polyimides made them particularly useful in the semiconductor industry. They can be used, for example, as dielectric layers, alpha particle barriers in memory devices, ion implantation masks and passivation layers. The goal of numerous development programs has been the development of a simple, reliable and cost effective radiation sensitive polyimide system that could be used with common photolithographic equipment and processes. This was the goal of by the work of R. Rubner et al of Siemens Co. (R. Rubner, H. Ahne, E Kuhn, G. Kolodziej; Phot. Sci. 4 Eng. 23(5), 303-309 (1979). H. Ahne, H. Kruger, E. Pammer and R. Rubner, "Polyimide Synthesis, Characterization and Application", K. L. Mittal, ed., Vol. 2, 905-918, Plenum Press (1984). The basic systems in these publications consist of polyamic acid polymer bearing photoreactive side groups. To date, however, materials based on this chemistry have been plagued by poor shelf life, extraordinarily low photospeed and excessive post-development/post-cure structure contraction. Although materials based on this chemistry have yielded high resolution structures, they required exposures of ten minutes or longer. Shelf life was also notoriously short, particularly in highly concentrated solutions required for thick film applications and contraction of original structures upon cure was as much as 60 percent.
The present invention provides the synthesis of the novel hydroxy polyimides, their application as radiation sensitive composition; e.g., an alkali soluble hydroxy polyimide and a quinone diazide type sensitizer. In addition the polyimides of the invention can be used to provide high temperature protective coating.
High temperature resistant relief patterns with excellent resolution and adhesion properties can be made from the radiation-sensitive compositions of the present invention.